Re: [RFC PATCH 2/2] mm, fs: daxfile, an interface for byte-addressable updates to pmem

On Wed, Jun 21, 2017 at 09:53:46AM +1000, Dave Chinner wrote:

On Tue, Jun 20, 2017 at 09:17:36AM -0700, Dan Williams wrote:

quoted

On Tue, Jun 20, 2017 at 1:49 AM, Christoph Hellwig [off-list ref] wrote:

quoted

[stripped giant fullquotes]

On Mon, Jun 19, 2017 at 10:53:12PM -0700, Andy Lutomirski wrote:

quoted

But that's my whole point.  The kernel doesn't really need to prevent
all these background maintenance operations -- it just needs to block
.page_mkwrite until they are synced.  I think that whatever new
mechanism we add for this should be sticky, but I see no reason why
the filesystem should have to block reflink on a DAX file entirely.

Agreed - IFF we want to support write through semantics this is the
only somewhat feasible way.  It still has massive downsides of forcing
the full sync machinery to run from the page fauly handler, which
I'm rather scared off, but that's still better than creating a magic
special case that isn't managable at all.

An immutable-extent DAX-file and a reflink-capable DAX-file are not
mutually exclusive,

Actually, they are mutually exclusive: when the immutable extent DAX
inode is breaking the extent sharing done during the reflink
operation, the copy-on-write operation requires allocating and
freeing extents on the inode that has immutable extents. Which, if
the inode really has immutable extents, cannot be done.

That said, if the extent sharing is broken on the other side of the
reflink (i.e. the non-immutable inode created by the reflink) then
the extent map of the inode with immutable extents will remain
unchanged. i.e. there are two sides to this, and if you only see one
side you might come to the wrong conclusion.

However, we cannot guarantee that no writes occur to the inode with
immutable extent maps (especially as the whole point is to allow
userspace writes and commits without the kernel being involved), so
extent sharing on immutable extent maps cannot be allowed...

Just to play devil's advocate...

/If/ you have rmap and /if/ you discover that there's only one
IOMAP_IMMUTABLE file owning this same block and /if/ you're willing to
relocate every other mapping on the whole filesystem, /then/ you could
/in theory/ support shared daxfiles.

However, that's so many on-disk metadata lookups to shove into a
pagefault handler that I don't think anyone in XFSland would entertain
such an ugly fantasy.  You'd be making a lot of metadata requests, and
you'd have to lock the rmapbt while grabbing inodes, which is insane.

Much easier to have a per-inode flag that says "the block map of this
file does not change" and put up with the restricted semantics.

--D

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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